Replacement source/drain finFET fabrication

ABSTRACT

A finFET is formed having a fin with a source region, a drain region, and a channel region between the source and drain regions. The fin is etched on a semiconductor wafer. A gate stack is formed having an insulating layer in direct contact with the channel region and a conductive gate material in direct contact with the insulating layer. The source and drain regions are etched leaving the channel region of the fin. Epitaxial semiconductor is grown on the sides of the channel region that were adjacent the source and drain regions to form a source epitaxy region and a drain epitaxy region. The source and drain epitaxy regions are doped in-situ while growing the epitaxial semiconductor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/192,378, filed Jul. 27, 2011, which is incorporated herein by thisreference in its entirety.

BACKGROUND

1. Field

The present disclosure relates generally to fabricating fin field effecttransistors (finFETs), and, more specifically, to fabricating finFETswith replacement sources and drains.

2. Description of Related Art

A finFET is a non-planar multi-gate transistor that operates on the sameprinciples as traditional metal oxide semiconductor field effecttransistors (MOSFETs). An island or fin of silicon is first patterned onto the wafer. After forming the fin, a gate stack is deposited andpatterned so that the patterned gate material runs perpendicular to thefin. Where the patterned gate material overlaps the fin, a gate for thefinFET is formed. The finFET will have one gate on both verticalsidewalls of the fin. Depending on the size of the top surface of thefin, the finFET may also have a gate on top of the fin.

For example, FIG. 1 depicts finFET 100 formed on a silicon-on-insulator(SOI) wafer. Fin structure 102 protrudes from oxide layer 108 and israised above shallow trench isolation (STI) 106. Gate stack 104 contactsfin structure 102 on three sides forming multiple gates. A channelregion is defined by the region under gate stack 104 in fin structure102. Source and drain regions are adjacent the channel region atopposite ends in fin structure 102.

FIG. 2 depicts traditional MOSFET 200 formed on oxide layer 208 of anSOI wafer. In contrast to finFET 100 (FIG. 1), silicon region 202 isplanar with STI 206, which causes gate stack 204 to contact only asingle side of silicon region 202 and form only a single gate.

The multiple gates of finFETs may offer many improvements overtraditional MOSFETs. For example, finFETs may be more robust againstshort-channel effects and may offer higher current drive while improvingsubthreshold swing.

However, the non-planar nature of finFETs may lead to severaldifficulties during fabrication. For example, conventional implantersrequire line of sight to implant ions into a surface. To dope verticalsidewalls of the source or drain of a finFET, the wafer must betilt-angle implanted. To overcome shadowing effects from nearby finFETs,the implant may need to be performed at multiple angles or orientations,which may increase implanter tool complexity, implanting cost, andprocessing time. Alternatively, design rules may be implemented thatspace non-planar structures far enough apart to minimize shadowingeffects. However, using design rules to increase structure spacing willresult in less dense circuits.

One alternative to tilt-angle implants is plasma doping. However, plasmadoping may have its own set of disadvantages, such as doping control andlimits on doping concentrations.

SUMMARY

In an exemplary embodiment, a finFET is formed having a fin with asource region, a drain region, and a channel region between the sourceand drain regions. The fin is etched on a semiconductor wafer. A gatestack is formed having an insulating layer in direct contact with thechannel region and a conductive gate material in direct contact with theinsulating layer. The source and drain regions are etched leaving thechannel region of the fin. Epitaxial semiconductor is grown on the sidesof the channel region that were adjacent the source and drain regions toform a source epitaxy region and a drain epitaxy region. The source anddrain epitaxy regions are doped in-situ while growing the epitaxialsemiconductor. The source and drain epitaxy regions may apply a strainto the channel region to improve the mobility of electrons or holes inthe channel region.

DESCRIPTION OF THE FIGURES

The present application can be best understood by reference to thefollowing description taken in conjunction with the accompanyingfigures, in which like parts may be referred to by like numerals.

FIG. 1 depicts a finFET.

FIG. 2 depicts an SOI MOSFET.

FIG. 3 depicts a flow chart of an exemplary process for forming afinFET.

FIG. 4A depicts a perspective view an exemplary embodiment of a finFETat an initial stage of fabrication according to the exemplary processdepicted in FIG. 3.

FIG. 4B depicts a cross-section view of the exemplary embodiment alongdotted cut plane 402 in FIG. 4A.

FIG. 4C depicts another cross-section view of the exemplary embodimentalong dotted cut plane 404 in FIG. 4A.

FIGS. 5A-5C; 6A-6C; 7A-7C; 8A-8C; 9A-9C; and 10A-10C depict theexemplary embodiment at various stages of fabrication according to theexemplary process depicted in FIG. 3.

FIGS. 11A-11C depict an exemplary embodiment of a finFET on an SOIwafer.

FIGS. 12A-12C depict an exemplary embodiment of a finFET withasymmetrical source and drain regions.

FIGS. 13A-13C depict a stage of another exemplary process forfabricating a finFET.

FIGS. 14A-14C depict a stage of another exemplary process forfabricating a finFET.

FIGS. 15A-15C depict a stage of another exemplary process forfabricating a finFET.

The figures depict various embodiment of the present invention forpurposes of illustration only. One skilled in the art will readilyrecognize from the following discussion that alternative embodiments ofthe structures and methods illustrated herein can be employed withoutdeparting from the principles of the invention described herein.

DETAILED DESCRIPTION

The following description is presented to enable a person of ordinaryskill in the art to make and use the various embodiments. Descriptionsof specific devices, techniques, and applications are provided only asexamples. Various modifications to the examples described herein will bereadily apparent to those of ordinary skill in the art, and the generalprinciples defined herein may be applied to other examples andapplications without departing from the spirit and scope of the variousembodiments. Thus, the various embodiments are not intended to belimited to the examples described herein and shown, but are to beaccorded the scope consistent with the claims.

FIG. 3 depicts an exemplary process 300 for fabricating a finFET.Corresponding FIGS. 4A-4C; 5A-5C; 6A-6C; 7A-7C; 8A-8C; 9A-9C; and10A-10C depict the finFET at various stages of fabrication according toexemplary process 300 (FIG. 3).

In particular, FIGS. 4A-4C depict substrate 400, which is a portion of asemiconductor wafer. In the present exemplary embodiment, substrate 400is a portion of a bulk silicon wafer. However, other types of wafers orsubstrates may also be used, such as SOI wafers. Also, semiconductormaterials other than silicon may be used, including but not limited toGe, SiGe, SiC, GeP, GeN, InGaAs, GaAs, InSb, InAs, GaSb, and InP.

With reference to FIG. 3, in operation 302, a fin structure is etched onthe substrate. FIGS. 5A-5C depict substrate 400 after formation of finstructure 500, which is directly connected to the silicon of the wafer.This operation may use standard semiconductor masking and etchingtechniques to form fin structure 500.

Although not required, an STI region may also be formed after formationof fin structure 500. FIGS. 5A-5C depict substrate 400 after formationof STI 502. After forming fin structure 500, the STI insulating materialmay be deposited over the substrate. Planarizing techniques such aschemical-mechanical polishing (CMP) or etch-backs may be used to removethe STI insulating material until top 504 of STI 502 is approximatelyplanar with top 506 of fin structure 500. While FIGS. 5A-5C show STI 502being perfectly planar with fin structure 500, in practice there may bea small step between these regions. Even with a small step, the regionsare still considered to be approximately planar. In other exemplaryprocesses, the STI formation may take place later in the processing. Forexample, the STI formation may take place after the source/drain regionshave been etched and refilled, as discussed below with respect to FIGS.14A-14C and 15A-15C.

If an STI region is formed, an etch-back may also be used to expose aportion of the fin structure. FIGS. 6A-6C depict substrate 400 after anetch-back of STI 502 to expose top portion 600 of fin structure 500.Bottom portion 602 of fin structure 500 remains covered by STI 502. Inone example, the STI insulating material is made of silicon dioxide. Inthis example, an etch that is selective to oxide over the silicon of thewafer may be used to etch STI 502 without etching significant amounts offin structure 500. Alternatively, a photoresist mask or hardmask may beused to protect fin structure 500 during the etch-back. In otherexemplary processes that form an STI region, for example as discussedbelow with reference to FIGS. 12A-12C, the etch-back may alternativelybe performed later in the process after the source/drain regions havebeen etched and refilled.

With reference back to FIG. 3, in operation 304, the gate of the finFETis formed by depositing and patterning a gate stack. FIGS. 7A-7C depictsubstrate 400 after a gate stack has been deposited and patterned toform a gate. The deposition of the gate stack starts with growing ordepositing gate dielectric 700 over substrate 400. FIGS. 7A-7C depictgate dielectric 700 being present over fin structure 500 and STI 502.Accordingly, in the case of FIGS. 7A-7C, gate dielectric 700 is either adeposited material or a material that may be grown on silicon as well asthe insulating material of STI 502. If gate dielectric 700 is thermaloxide, the portion of gate dielectric 700 over STI 502 will not bepresent.

Next, conductive gate material 702 is deposited. In one example,conductive gate material 702 is poly or amorphous silicon, which may beimplanted to reduce resistance and set the gate workfunction. Otherconductive gate materials, such as metals, may also be used.

In other exemplary embodiments, the gate stack may also be a high-k,metal gate (HKMG) stack. For example, a metal gate, such as titaniumnitride, with high-k gate dielectrics, such as such as hafnium dioxide(HfO₂), zirconium dioxide (ZrO₂), or titanium dioxide (TiO₂), may beused. The HKMG stack may be formed with a dielectric-first, gate-first,dielectric-last, or gate-last process.

For example, in a dielectric-last HKMG process, the gate stack may beinitially formed as a traditional silicon dioxide and polysilicon stackthat is used as a sacrificial stack during front-end of line processing.Then, before proceeding to back-end of line processing, a portion ofthis sacrificial stack may be removed and replaced with a gate stackthat includes a high-k dielectric and a metal gate. This is also anexample of a gate-last HKMG process.

In another example of a gate-last HKMG process, the gate dielectric maybe formed with a high-k material (with or without traditionaldielectrics) and polysilicon for the gate material. Then, beforeproceeding to the back-end of line processing, the polysilicon gate willbe removed and replaced with a metal gate. The gate dielectric depositedearlier in the front-end of line processing will remain. This is also anexample of a dielectric-first HKMG process.

In a gate-first, dielectric-first HKMG process, both the high-k gatedielectric and the metal gate are formed during the front-end of lineprocessing.

After the gate stack is deposited, hardmask layer 704 is deposited andpatterned. The patterned hardmask may used to etch conductive gatematerial 702 and optionally gate dielectric 700. The remaining gatestack material forms gate 706. Channel 708 is the region in finstructure 500 that is approximately covered by gate 706. Source anddrain 710 are in either end of fin structure 500 adjacent to channel708. In a discrete device as is being described, the source and drainare described together as they are interchangeable.

In other processes, the hardmask may be omitted if a photoresist mask isused to pattern gate 706. The gate dielectric may also be left unetchedover the entire wafer. It may be removed, for example, in the spaceretch step as described with respect to FIGS. 8A-8C.

With reference to FIG. 3, in operation 306, some or all of thesource/drain regions are etched leaving the channel region under thegate of the fin structure remaining. In one example of this operation,the gate hardmask, along with a spacer, may be used as the mask for thesource/drain etch. FIGS. 8A-8C depict substrate 400 after the formationof spacer 800 and the etching of source and drain 710 (FIG. 7A) toremove fin structure 500 in those regions. The formation of spacer 800may start with deposition of an insulating spacer material oversubstrate 400. A blanket spacer etch may then be performed. The spacerinsulating material on the sidewalls of gate 706 will be protected fromthe etch. After the etch is completed by, for example, a set time orend-point, spacers 800 will remain. Additionally, hardmask layer 704,left on the wafer after patterning the gate, may still remain after thespacer insulating material is etched away. Hardmask layer 704 and spacer800 may then be used as a mask to etch source and drain 710 (FIG. 7A).After the source and drain etch, most or all of the silicon from sourceand drain 710 (FIG. 7A) is removed, leaving channel 708 of fin structure500.

Operation 306 may also omit the spacer related operations. This versionof operation 306 may use only the hardmask to etch the source and drainregions. In other example of operation 306, a photoresist mask may usedinstead of a hardmark.

With reference to FIG. 3, in operation 308, portions of the source anddrain that were etched in operation 306 are refilled with silicon. FIGS.10A-10C depict substrate 400 after etched source and drain 710 (FIG. 7A)have been refilled with silicon 1000. This may be done, for example, bydepositing polysilicon or growing epitaxial silicon. If the sidewalls ofchannel 708 have been damaged during the source and drain etch, thesidewalls may optionally be prepared to receive new silicon by, forexample, thermally oxidizing the sidewalls to consume damaged silicon.Even though source and drain 710 (FIG. 7) were silicon, othersemiconductors may be used to refill the regions. In this case, thechannel region and the source/drain regions would be made of differentmaterials.

With reference to FIG. 3, in operation 310, which occurs concurrentlywith operation 308, the source and drain are doped in-situ while beingrefilled with silicon. With reference again to FIGS. 10A-10C, by dopingwhile depositing or growing silicon 1000, tilt angle and plasma implantsmay be avoided. Additionally, because in-situ doping allows for aconstant heavy doping across silicon 1000, the source and drainresistances may be reduced as compared to implants alone. The thermalsteps involved with activating any source and drain implants may also beavoided.

If a blanket silicon deposition or growth is used, an additional etchstep may be needed to remove silicon from non-source and non-drainareas. CMP or an etch-back (with or without a masking step) may be usedto remove silicon from regions that should be non-conducting (e.g., toremove any silicon that may be shorting the source and drain). Forexample, as depicted in FIGS. 10A-10C, silicon 1000, which was depositedto refill previously etched source and drain 710 (FIG. 7A), has beenetched back so that the top of gate 706 is exposed and the source anddrain are not shorted together.

While silicon 1000 of FIGS. 10A-10C is depicted with one particularshading, the crystalline structure of silicon 1000 may vary across thedevice. For example, if silicon 1000 is produced with an epitaxialprocess, the portion of silicon 1000 in the source and drain regions maybe crystalline while the portion of silicon 1000 over the STI may bepolycrystalline. The use of a single shading for silicon 1000 is notintended to indicate that silicon 1000 has a uniform crystallinestructure or necessarily has other uniform characteristics.

Alternatively, selective epitaxial growth may be used to grow silicononly on exposed silicon (e.g. silicon portions of the wafer and fin thatare exposed after removal of the source and drain in operation 306) andnot on other materials (e.g., the STI insulator). FIGS. 9A-9C depictsubstrate 400 after selectively growing silicon 900 to refill etchedsource and drain 710 (FIG. 7A). With selective epitaxy, an etch-backstep may not be necessary. FIGS. 9A-9C are in contrast to FIGS. 10A-10C,which depict substrate 400 after non-selective growth and an etch-backstep. The shape of source and drain 900 in FIGS. 9A-9C is only intendedto be exemplary of a shape produced by using selective epitaxy to regrowthe source and drain regions. Other shapes of the source and drainregions may also be produced without deviating from the exemplaryembodiments of this invention.

To control the thermal budget, initially, a high-temperature epitaxyprocess may be used to grow an initial thickness of high qualitysilicon. Polysilicon may then be deposited at a lower temperature torefill more of the source and drain. CMP, an etch-back, or both may alsobe used to plannerize the wafer. Additionally, an etch-back may berequired to remove silicon from non-source and non-drain regions, asdescribed above.

While FIGS. 9A-9C and 10A-10C depict the etched source and drain beingcompletely refilled with silicon, in other cases, only a portion of thesource and drain needs to be refilled with silicon.

In addition to creating a low-resistance source and drain, thereplacement of source and drain 710 (FIG. 7A) may also add strain tochannel 708 (FIG. 10A). For example, SiGe or SiC (if using a siliconsubstrate) may be grown with selective epitaxy on the sides of channel708 (FIG. 10A) where the source and drain regions were etched away. Thedifferent lattice constants of SiGe or SiC as compared to Si (or othersemiconductor material forming the channel) may strain the semiconductorin channel 708, which may increase the mobility of electrons or holes inchannel 708.

While the first exemplary process was described with respect to severalprocessing steps, those skilled in the art will recognize that otherwell-known processing steps will also be required to create afunctioning finFET. For example, threshold adjust implants may be neededto properly set the threshold voltage for n-type or p-type finFETs. Asanother example, the refilling of the source and drain throughdeposition or epitaxial growth of silicon may need to be performedtwice: once for p-type finFETs requiring p-type sources and drains andonce for n-type finFETs requiring n-type sources and drains.

FIGS. 11A-11C depict another exemplary embodiment of a finFET onsubstrate 1100. This exemplary embodiment is similar to the exemplaryembodiment depicted in FIGS. 10A-10C, except that an SOI wafer is usedfor substrate 1100 instead of a bulk wafer as described above. Also, theprocess used for this exemplary embodiment does not include formation ofan STI region. The shape of source and drain 1102 in FIGS. 11A-11C isonly intended to be exemplary of a shape produced by using selectiveepitaxy to regrow the source and drain region. Other shapes of thesource and drain regions may also be produced without deviating fromembodiments of the invention.

FIGS. 12A-12C depict another exemplary embodiment of a finFET withasymmetrical source 1200 and drain 1202 that have different materialproperties. For example, source 1200 and drain 1202 may be made ofdifferent materials. As another example, the doping of the two regionscould be different. In yet another example, the strain of the tworegions could be different. This exemplary embodiment requires source1200 and drain 1202 to be grown separately. The shape of source 1200 anddrain 1202 in FIGS. 12A-12C is only intended to be exemplary of a shapeproduced by using selective epitaxy to regrow the source and drainregion. Other shapes of the source and drain regions may also beproduced without deviating from embodiments of the invention.

FIGS. 13A-13C depict a stage of another exemplary process. Thisexemplary process is similar to exemplary process 300 (FIG. 3), exceptthat the STI etch-back step described above with respect to FIGS. 5A-5Cis performed later in the process. FIGS. 13A-13C depict substrate 1300after formation of fin structure 1306 (described in operation 302),deposition of STI 1304 (described with respect to FIGS. 5A-5C), andetching and refilling of source drain 1302 (described in operations 306,308, and 310). The etch-back of STI 1304, however, has not occurred yet.The STI etch-back step (described with respect to FIGS. 5A-5C) occurs atsome point prior the gate formation (operation 304). Substrate 1300 isdepicted as an SOI wafer; however, this process can be used on a bulkwafer as well.

FIGS. 14A-14C depict a stage of another exemplary process forfabrication of a finFET. This exemplary process is similar to exemplaryprocess 300 (FIG. 3), except the source and drain etch and refill(described in operations 306, 308, and 310) takes place prior toformation of the fin structure (described in operation 302). In thisprocess, a standard photoresist mask may be used to etch away source anddrain 1402 prior to refilling. Additionally, only silicon that will formsource and drain 1402 are etched and refilled. Region 1404 remainsunetched and is the original top surface of substrate 1400. A subsequentstage in this exemplary process forms a fin structure as described abovewith respect to operation 302. Substrate 1400 is depicted as an SOIwafer; however, this process can be used with a bulk wafer as well.

FIGS. 15A-15C depict a stage of another exemplary process forfabrication of a finFET. This exemplary process is similar to theexemplary process described above with reference to FIGS. 14A-14C,except in addition to source and drain 1502 being etched and refilled,area 1504 around source and drain 1502 are also etched and refilled.Only region 1506 remains of the original top surface of substrate 1500.A subsequent stage in this exemplary process forms a fin structure asdescribed above with respect to operation 302. Substrate 1500 isdepicted as an SOI wafer; however, this process can be used with a bulkwafer as well.

While source and drain 1502 and area 1504 around source and drain 1052of FIGS. 15A-15C are depicted with one particular shading, thecrystalline structure of these areas may vary across the device. Forexample, if epitaxy is used to regrow silicon in these areas, thesilicon adjacent region 1506 may be crystalline while the siliconfurther away from region 1506 may be polycrystalline. The use of asingle shading is not intended to mean that these areas have uniformcrystalline structure or necessarily have other uniform characteristics.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and it should be understoodthat many modifications and variations are possible in light of theabove teaching.

We claim:
 1. A method for fabricating a fin field effect transistor(finFET) with a fin having a source region, a drain region, and achannel region between the source and drain regions, the methodcomprising: etching a semiconductor substrate to form a semiconductorbody on the semiconductor substrate and cavities on opposite sides ofthe semiconductor body; growing epitaxial semiconductor on oppositesides of the semiconductor body to at least partially fill the cavities;forming a fin having a source region, a drain region, and a channelregion by etching the semiconductor body and the grown epitaxialsemiconductor, wherein a long dimension of the semiconductor body priorto forming the fin is greater than a width of the fin; and forming agate stack on the channel region, the gate stack having an insulatinglayer disposed on the channel region, and a gate material disposed onthe insulating layer.
 2. The method of claim 1 further comprising:depositing an isolation layer around and on the fin; and polishing theisolation layer to have a top surface approximately coplanar with a topsurface of the fin.
 3. The method of claim 2, wherein depositing theisolation layer and polishing the isolation layer are performed prior toforming the gate stack.
 4. The method of claim 3, further comprising:etching the isolation layer back so that a top portion of the fin isexposed while the isolation layer covers a bottom portion of the fin. 5.The method of claim 1, further comprising: growing thermal oxide onopposite sidewalls of the semiconductor body to consume damage to theopposite sidewalls caused by etching to form the semiconductor body. 6.The method of claim 1, wherein the source region and the drain regionare configured to add strain to at least a portion of the channel regionto improve the mobility of electrons or holes in the channel region. 7.The method of claim 1, wherein the source region comprises a firstepitaxial semiconductor and the drain region comprises a secondepitaxial semiconductor that is different from the first epitaxialsemiconductor.
 8. The method of claim 1, wherein the channel regioncomprises a different material than the source region and the drainregion.
 9. The method of claim 1, wherein growing epitaxialsemiconductor comprises growing epitaxial semiconductor selectively to amaterial of the semiconductor substrate.
 10. The method of claim 1,wherein growing epitaxial semiconductor comprises doping the epitaxialsemiconductor in-situ.
 11. The method of claim 1, wherein the channelregion comprises a portion of the semiconductor body.
 12. The method ofclaim 11, wherein the source region and drain region are disposed onopposite sides of the channel region, and wherein the source region anddrain region each comprise a portion of the grown epitaxialsemiconductor.
 13. A method for fabricating a fin field effecttransistor (finFET), the method comprising: etching a semiconductorsubstrate to form a semiconductor body on the semiconductor substrateand a pair of cavities disposed on opposite sides of the semiconductorbody; growing a first epitaxial semiconductor on a first side of thesemiconductor body to at least partially fill a first cavity of the pairof cavities; growing a second epitaxial semiconductor on a second sideof the semiconductor body opposite to the first side to at leastpartially fill a second cavity of the pair of cavities, wherein thefirst epitaxial semiconductor is different from the second epitaxialsemiconductor; forming a fin having a source region, a drain region, anda channel region by etching the semiconductor body, the first epitaxialsemiconductor, and the second epitaxial semiconductor, wherein a longdimension of the semiconductor body prior to forming the fin is greaterthan a width of the fin; and forming a gate stack on the channel region,the gate stack having an insulating layer disposed on the channelregion, and a gate material disposed on the insulating layer.
 14. Themethod of claim 13, further comprising: growing thermal oxide onopposite sidewalls of the semiconductor body to consume damage to theopposite sidewalls caused by etching to form the semiconductor body. 15.The method of claim 13, wherein the source region and the drain regionare configured to add strain to at least a portion of the channel regionto improve the mobility of electrons or holes in the channel region. 16.The method of claim 13, wherein a material of the semiconductor body isdifferent from the first epitaxial semiconductor and the secondepitaxial semiconductor.
 17. The method of claim 13, wherein the firstepitaxial semiconductor and the second epitaxial semiconductor are grownselectively to a material of the semiconductor substrate.
 18. The methodof claim 13, wherein growing the first epitaxial semiconductor and thesecond epitaxial semiconductor comprises doping the first epitaxialsemiconductor and the second epitaxial semiconductor in-situ.
 19. Themethod of claim 13, wherein the channel region comprises a portion ofthe semiconductor body.
 20. The method of claim 19, wherein the sourceregion and drain region are disposed on opposite sides of the channelregion, wherein the source region comprises a portion of the grown firstepitaxial semiconductor, and wherein the drain region comprises aportion of the grown second epitaxial semiconductor.